Video Ranging

There are many situations in marine and aquatic wildlife management in which it is necessary to measure the range to animals or object seen floating at the water's surface.  Once of the most obvious is during abundance surveys of marine mammals or swimming birds using distance-sampling techniques, such as line transect surveys.  Other applications include measuring ranges to animals during behavioural studies or as part of mitigation requirements (for example determining whether animals are too close during potentially harmful activates such as seismic surveys) and measuring the shape and extent of flocks of swimming birds or floating objects such as oil slicks.

Estimating distance at sea by eye is notoriously difficult but one straightforward method for determining the range to objects at the surface is to measure the angle subtended between the waterline of an object and the horizon, when viewed from a known elevated height.  Navigators have used this technique for centuries, making accurate angular measurements using instruments such as sextants.  Sextants however are not suitable when the target is difficult to see and is only fleetingly visible, as is the case with a surfacing cetacean.  Boat based biologists have employed a variety of less sophisticated instruments to make these measurements, including sightings sticks, vernier callipers and reticule binoculars.  However the accuracy of such instruments, from a moving vessel at sea, is limited.

Jonathan Gordon's has pioneered a technique, where the required angles are simply  measured from photographic or video images that show both the target object and the horizon, have dramatically increased the accuracy with which range can be measured.  

• Estimating range to objects at sea by eye is notoriously difficult yet there are many occasions in management and research when accurate measures of range are required
• A new method is described in which range is calculated from the angle subtended between the horizon and the waterline of the object measured from a single video or photographic image taken at a  known elevation.  Possible errors are explored and practical analysis methods outlined Images can be collected and analysis performed using relatively inexpensive standard equipment.

Analysis programs will be available for download from this site shortly

Download notes on fitting a camera to big eye binoculars 

Download notes on setting up a Canon XM Digital Camcorder 

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Last modified: August 18, 2004